The present invention relates to nonaqueous electrolyte secondary cells which comprise a can and a rolled-up electrode unit housed in the can and serving as an electricity generating element, and more particularly to nonaqueous electrolyte secondary cells which are adapted to promptly suppress a current exceeding a predetermined value when the excessive current occurs.
In recent years, attention has been directed to lithium secondary cells or batteries which are adapted for a greater capacity and higher energy density for use as power sources for electric motor vehicles or hybrid cars. For example, FIGS. 5 and 6 show a cylindrical lithium secondary cell which comprises a cylindrical cell can 1 having a cylinder 11 and lids 12, 12 welded to the respective ends thereof, and a rolled-up electrode unit 2 encased in the cell can 1. A pair of positive and negative electrode terminal assemblies 9, 9 are attached to the lids 12, 12, respectively. The rolled-up electrode unit 2 is connected to the terminal assemblies 9, 9 by a plurality of current collector tabs 3, whereby the electric power generated by the electrode unit 2 can be delivered to an external device from the pair of terminal assemblies 9, 9. Each lid 12 is provided with a gas vent plug 13.
With reference to FIG. 7, the rolled-up electrode unit 2 comprises a positive electrode 23 containing a lithium containing composite oxide, a negative electrode 21 containing a carbon material, and a separator 22 impregnated with a nonaqueous electrolyte and interposed between the electrodes, the assembly of these components 21 to 23 being rolled up into a cylinder. A plurality of current collector tabs 3 outwardly extend from each of the positive electrode 23 and the negative electrode 21 of the unit 2, and the outer ends 31 of the current collector tabs 3 of the same polarity are joined to one electrode terminal assembly 9. For convenience"" sake, only some of these tabs are shown as being joined at their outer ends to the terminal assembly 9 in FIG. 6, while the outer ends of the other tabs connected to the assembly 9 are omitted from the illustration.
The electrode terminal assembly 9 comprises a screw member 91 extending through a hole in the lid 12 of the cell can 1 and mounted on the lid 12. The screw member 91 has a flange 92 at its base end. An insulating packing 93 is fitted in the hole of the lid 12 for electrical insulation and effective sealing. The screw member 91 has a washer 94 fitted therearound from outside the cylinder 11, and a first nut 95 and a second nut 96 screwed thereon similarly. The first nut 95 is tightened up to clamp the insulating packing 93 between the flange 92 of the screw member 91 and the washer 94 and thereby seal off the hole more effectively. The outer ends 31 of the current collector tabs 3 are secured to the flange 92 of the screw member 91 by laser welding or ultrasonic welding.
For connecting the current collector tab 3 to the negative electrode 21 or positive electrode 23 of the rolled-up electrode unit 2, a current collector strip forming the electrode and coated with an electrode material over a surface has a known structure comprising a portion of the surface not coated with the electrode material and having the base portion of the current collector tab secured thereto by laser welding or ultrasonic welding (JP-A No. 267528/1994).
When the nonaqueous electrolyte secondary cell described develops a short-circuit in its interior, a great current is likely to flow. To avoid such an incidence, an electrode structure has been proposed. For example, a positive electrode 8 comprises, as shown in FIG. 8, a current collector 81 which is provided with an electrode material 83 over each of its opposite surfaces, with a PTC (positive temperature coefficient) element layer 82 formed therebetween (JP-A No.220755/1995). The PTC element providing the layer 82 has a positive temperature coefficient of resistance, such that when a current in excess of a predetermined value flows therethrough, the electric resistance value of the element rapidly increases to exhibit a current suppressing effect. When the secondary cell having the proposed electrode structure develops an inside short-circuit, a current exceeding the predetermined value will not flow continuously.
However, in the case of the conventional nonaqueous electrolyte secondary cell having the PTC element layer 82 shown in FIG. 8, the presence of the PTC element layer 82 between the current collector 81 and the electrode material 83 makes the quantity of the electrode material 83 correspondingly smaller than otherwise per unit volume of the cell can, consequently entailing the problem of greatly reducing the discharge capacity per unit volume of the cell can, i.e. energy density.
An object of the present invention is to provide a nonaqueous electrolyte secondary cell which is adapted to prevent continuous occurrence of a current exceeding a predetermined value and realize a high energy density.
The present invention provides a nonaqueous electrolyte secondary cell which has an electrode unit 2 housed in a cell can 1 and comprising a positive electrode and a negative electrode each including a current collector. The cell is characterized in that the current collector of at least one of the positive electrode and the negative electrode comprises a plurality of current collector pieces 42 arranged along one direction and a PTC element 5 interconnecting each pair of adjacent current collector pieces 42.
With the secondary cell of the invention stated above, the PTC element 5 is interposed between each pair of adjacent current collector pieces 42 and can therefore be given a minimum length necessary for interconnecting the collector pieces 42. This greatly reduces the volume occupied by such PTC elements 5 in the interior of the cell can 1 unlike the conventional construction wherein a PTC element layer is interposed between the current collector and the electrode material, consequently permitting use of a larger amount of electrode material than conventionally to result in an energy density as high as is the case with cells having no PTC element.
Stated more specifically, end portions of the pair of current collector pieces 42, 42 to be interconnected are lapped over each other, with the PTC element 5 held therebetween, and joined to respective opposite surfaces of the PTC element 5. With this specific structure, the joint between the PTC element 5 and each current collector piece 42 can be given a sufficiently large area, whereby the adjacent current collector pieces 42 can be connected to each other firmly.
The number A of current collector pieces 42, the overall length B of the electrode along the winding direction of the electrode unit 2 and the length C of the PTC element 5 have the relationship represented by the following expression:
Axc3x97C/B less than 0.1
When this relationship is established, a much greater discharge capacity than in the prior art is available as will be substantiated by experimental results to be described later.
The present invention further provides another nonaqueous electrolyte secondary cell including an electrode unit 2 housed in a cell can 1 and comprising a positive electrode and a negative electrode each provided by coating a surface of a striplike current collector 61 with an electrode material 62 to form coated portions and an uncoated portion not coated with the electrode material 62, a current collector tab 3 having a base end portion connected to the uncoated portion and an outer end portion connected to an electrode terminal assembly 9. At least one of the positive electrode and the negative electrode is provided with a PTC element 7 held between opposed faces of the uncoated portion of the current collector 61 thereof and the base end portion of the current collector tab 3.
With the secondary cell of the invention described above, the PTC element 7 is provided between the uncoated portion of the current collector 61 and the current collector tab 3 and can therefore be given a length equal to the width of the current collector tab 3. This greatly reduces the volume occupied by such PTC elements 7 in the interior of the cell can 1 unlike the conventional construction wherein a PTC element layer is interposed between the current collector and the electrode material, consequently permitting use of a larger amount of electrode material than conventionally to result in an energy density as high as is the case with cells having no PTC element.
Stated specifically, the PTC element 7 has a thickness in the range of 10 xcexcm to 500 xcexcm. When the PTC element 7 has a thickness in this range, the cell has a sufficient current suppressing effect and a greater discharge capacity than in the prior art as will be substantiated by experimental results to be described later.
The present invention further provides another nonaqueous electrolyte secondary cell which includes a plurality of current collector tabs each of which is partly or entirely made of a PTC element. When the current flowing through one of the PTC element portions constituting the current collector tabs in the secondary cell increases or one of the PTC element portions rises in temperature, the electric resistance of the particular PTC element portion rapidly increases, permitting little or no current to flow through the portion, whereby the cell current is cut off.
Proposals have been made of a secondary cell wherein a single PTC element is connected between the positive electrode and the positive electrode external terminal (JP-U No. 53929/1977), and a secondary cell having an opening closure plate which is made of a PTC element (JP-A No. 74493/1993). However, since the allowable current value of one PTC element is, for example, as small as about 5 A, these secondary cells can not be charged with a great current, and the PTC element is not usable in large secondary cells.
On the other hand, the nonaqueous electrolyte secondary cell of the invention, wherein the plurality of PTC elements are connected in parallel, can be charged with a great current having a value obtained by multiplying the allowable current value of the PTC element by the number of PTC elements, so that the invention can be embodied as large secondary cells.
Further when the rolled-up electrode unit of the conventional secondary cell described locally rises in temperature, the conduction of heat from the location of rise of temperature to the PTC element takes time, hence the problem of delayed current cutoff.
With the secondary cell embodying the present invention, the electrode unit is, for example, a rolled-up electrode unit wherein a plurality of PTC elements are arranged as distributed longitudinally of each of the positive and negative electrodes of the unit. If a rise in temperature occurs at a portion along the length of the positive or negative electrode, the electric resistance of the PTC element in the vicinity of the portion rapidly increases to block the current through the PTC element almost completely. Thus, the current is cut off promptly.
The nonaqueous electrolyte secondary cell embodying the invention is adapted to prevent continuous occurrence of a current exceeding a predetermined value and is given a high energy density as already stated.